Conventional ignition plugs for igniting fuel (air-fuel mixture) by means of plasma include plasma jet ignition plugs as disclosed in Japanese Patent Application Laid-Open (kokai) No. 2007-287665 and Japanese Patent Application Laid-Open (kokai) No. 2008-45449, and igniter plugs as disclosed in Japanese Patent Application Laid-Open (kokai) No. 3-214582.
For example, a plasma jet ignition plug has, at its front end portion, a cylindrical cavity surrounded by a center electrode and an insulator. When a spark discharge of high energy occurs between the center electrode and a ground electrode, the interior of the cavity instantaneously acquires an intense heat state. Then, an air-fuel mixture present in the cavity is ionized and, at the same time, rapidly expands, thereby jetting out from the cavity in the form of a flame-like plasma. Since such a flame-like plasma extends into a cylinder, the area of contact with the air-fuel mixture increases. Thus, the plasma jet ignition plug is characterized by superiority in ignition performance over an ordinary spark plug which ignites fuel by means of sparks.
However, a conventional plasma jet ignition plug requires relatively high discharge voltage for generating a spark discharge between the center electrode and the ground electrode before jetting of plasma. This involves a problem of increase in generated electric noise and a problem of deterioration in the cavity and a through-hole (orifice) of the ground electrode caused by the occurrence of channeling.
In order to solve these problems, for example, the igniter plug described in the aforementioned Japanese Patent Application No. 3-214582 employs a solid-type semiconductor chip disposed between the center electrode and the ground electrode so as to lower discharge voltage. However, such a structure may involve the occurrence of misfire caused by discharge between the center electrode and a metallic shell stemming from the flow of current in the vicinity of the interface between the semiconductor chip and the insulator.
The present invention overcomes the above-mentioned problems and provides an ignition plug which ignites fuel by means of plasma with low discharge voltage and exhibits high ignition performance.